1. Field of the Invention
The invention relates to a method of manufacturing an optical information carrier by recording information, comprising information units, in a master disc, in which a radiation beam scanning the master disc is modulated in power by the information and exposes a photoresist layer of the master disc to form a pattern of short and long exposed areas, and subsequently, the photoresist is developed to form a pattern of information areas, the beginning and duration of the exposure for each exposed area being determined by the associated information unit. The invention also relates to an arrangement for carrying out the method and to an optical information carrier obtained by carrying out the method.
An information unit is a portion of an information stream represented by a single feature on the master disc. For example, if a digital information signal alternately has a value zero and one and a signal portion having the value one results in an exposed area whose length is determined by the length of time for which the signal has the value one, this portion of the information signal is an information unit. An exposed area is called short if its length is smaller than twice the diameter of the radiation spot formed on the photoresist layer by the radiation beam, the diameter being the distance between two diametrally disposed points within the radiation spot where the local intensity is equal to half the maximum intensity in the radiation spot. For an Airy intensity profile formed by an objective lens, said diameter is equal to half the quotient of the wavelength of the radiation and the numerical aperture of the objective lens. An information area is an area on the master disc having properties which can be transferred to information carriers by a replication process, the corresponding areas on the information carriers being also referred to as information areas. Hereinafter, the term "area" without any further qualification refers to an exposed area in the photoresist, unless otherwise stated.
In the method of manufacturing an optical information carrier, the information is first inscribed in the master disc by exposure of the photoresist layer. The photoresist layer is subsequently developed, as a result of which information areas, for example, in the form of pits, are formed at the location of the exposed areas. Although hereinafter the information areas will often be referred to as pits, it will be obvious that these areas may also comprise hills. Subsequently, the master disc is provided with a metal layer. After the metal layer has been removed from the photoresist layer, it forms a die with a negative impression of the pattern of pits. The pattern of pits of the die is then transferred to a multitude of optical information carriers by a replication process such as, for example, a plastics molding process, which information carriers may subsequently be provided with one or more layers to make them reflecting and/or inscribable. In an information carrier which cannot be inscribed by the user, the pattern of pits represents the recorded user information. In an inscribable information carrier, the pattern of pits may represent not only user information but also tracking information by means of which a radiation beam, used for reading or writing information, can be guided over the information carrier.
The information density on the information carrier can be increased by arranging the pits closer to one another. However, more densely packed pits give rise to a deterioration of the quality of the signal generated when the information on the information carrier is read, as a result of which the reliability with which the information is read decreases. The deterioration of the quality inter alia manifests itself in increased jitter, i.e., an increase of irregular random variations of the signal. In order to enable an information signal to be derived from the information carrier with a sufficiently low jitter in the case of higher densities, the pits in the master disc should have a very accurately defined position and shape. One of the problems which limit an increase in information density is that with a constant power of the radiation beam during the recording of information in the master disc, the short pits appear to become narrower than the long pits.
2. Description of the Related Art
U.S. Pat. No. 5,040,165 discloses a method of manufacturing an optical information carrier, which aims to solve the above problem. The power of the radiation beam exposing the photoresist layer is chosen to be 25% to 100% higher for recording short areas than for recording long areas. However, tests conducted on information carriers manufactured by means of this method reveal that the envisaged reduction of jitter is not achieved. Moreover, the ratio between the power for recording short areas and that for recording long areas appears to depend strongly on the type and condition of the photoresist used on the master disc, so that the quality of the information carriers obtained from different master discs is different.